For the high speed expansion of a so-called air bag in a motor vehicle and for other pyrotechnic purposes, it is common to provide a heating element which can serve to ignite a pyrotechnic or explosive charge. The heating element, usually in the form of strip, can comprise a base body, a structured resistance layer or strip on the base body and contact fields or pads which are applied overlappingly to the two ends of the resistance strip. These contact elements or pads allow connection of an excitation circuit to the heating body or element so that when an electrical current is passed through the latter, the resistance element will ignite the charge.
Heating elements of this type can be fabricated by applying a glass or glass ceramic by a screen printing process to a base body with subsequent drying and sintering of the applied layer. The steps are repeated until the desired total thickness is achieved. Then the resistance paste is screen-printed onto the glass or glass-ceramic layer, dried and sintered. Finally a conductive paste is applied to the resistance strip by a screen printing method and is dried and sintered.
The firm Dynamit Nobel AG has for many years made available heating elements fabricated by a thin layer technique or by sputtering as igniters for military explosives and mining charges (see DE 2020016 A1). These types of heating elements can be used for automotive applications only at additional expense for circuitry designed to protect against misfiring or other drawbacks.
The firm LifeSparc Inc. and Auburn University have developed heating elements fabricated by layering techniques (thin layer sputtering) upon semiconductor substrates (U.S. Pat. Nos. 4,798,060 and 4,976,200) which also must be protected against external influences by additional circuitry, for example, diodes included in the semiconductive substrate if they are to be useful for automotive applications.
Schaffler & Co. (Austrian patent 405591 B) have developed a heating element utilizing thick layer technology.
This system can be used to fire pyrotechnical charges without the additional circuitry referred to above but does not satisfy the specifications which have been set forth for the automotive industry with respect to electrostatic discharge (ESD) and with respect to transient pulses while maintaining requisite electrical resistances (for example 2 ohms) and ignition delay (for example at most 2 ms).
The specifications which must be satisfied for automotive purposes are for example the USCAR specification (Chrysler, General Motors and Ford) and the VW80150 specification (of Volkswagen). Apart from requirements with respect to the environment (climate change tests and mechanical loading) the electrical requirements for the heating element (or example sensitivity to ignition and resistance characteristics in eh case of false pulses), is of the greatest significance. Tests are then carried out on such igniters so that the heating elements can satisfy the requirements of the automotive industry. In particular, the sensitivity to ignition can be determined by so-called “all fire” and “no fire” tests (for example Bruceton, Logit, Run-Down).
In the “all fire” test, the igniter must fire upon the application of a constant current pulse of 1.2 A within 2 ms to a certain statistical probability. In the “no fire” test the heating element must not fire under a constant current pulse of 0.5 A over a period of 10 seconds to a certain statistical probability. When the igniter receives a false pulse as described, it should not fire. A false pulse is a predetermined quantity of energy which can have a defined duration and a predetermined repetition frequency.
For example, an ESD false pulse in accordance with the USCAR standard is the discharge of a 150 pF condenser charged to 25 kV through a charging resistance of 500 ohm through the 2 ohm igniter heating element.
An example of a transient pulse in accordance with USCAR is a current pulse of 5.3 A with a pulse duration of 4 μs (rise time=1 μs, decay time=3 μs) and a repetition rate or keying ratio of 1:1000 over 24 hours through the 2 Ω heating elements.
The problem with igniters with all of the known heating elements is that they have been able to satisfy these specifications only with additional electronics. Up to now no heating element has been developed which could be fabricated by layering technology (thick layer, thin layer or semiconductor) which has satisfied the requirements of the automotive industry without the additional expense of external circuitry in accordance with the aforementioned specifications.